1. Field of the Invention
The present invention relates to an image forming apparatus comprising a heat fixing device used for electrophotography, such as a copier, a printer and a FAX. More particularly, the present invention relates to an image forming apparatus comprising a heat fixing device using a belt.
2. Description of Related Art
In a fixing device of electrophotography, a transfer medium supporting a toner image is passed through a nip portion that is formed simply by pressing two rollers and toner is fixed on the transfer media (9MA, 9MB and 9MC). As an earlier example, a device in which at least one of the two rotators is replaced by a belt is also developed.
FIGS. 4A to 4D are views showing outlines of fixing devices using a belt.
However, when the diameters of the two rollers forming a nip is made large in order to gain a nip portion 9NA width as shown in FIG. 4A, the curvatures of the two rotators become small at the transfer medium outlet of the nip portion 9NA and a problem that the transfer medium 9MA cannot be separated from the heating belt 9BA occurs. Accordingly, there is a limitation of accelerating the fixing process. On the other hand, fixing performance and separation performance are ensured in a technique that two or more rollers are provided inside the heating belt 9BB and a roller having a small diameter which contacts with the inside of the heating belt 9BB is provided at the downstream area of the nip portion 9NB to ensure the separation performance, as shown in FIG. 4B. However, there is problem that an area f where the heating belt 9BB is not supported by a roller is necessarily formed and image shift is caused. As above, in cases that the separation points of a transfer medium from two rotators are the same position, there is limitation on acceleration of the fixing process.
FIG. 4C shows a development in which the separation points of a transfer medium from two rotators are made different in order to solve the above problem.
In the development, a transfer medium 9MC is separated from a heating belt 9BC at the position of the maximum curvature of a separation roller 9SC for stretching the heating belt 9BC at the downstream in the paper feeding direction from a main nip portion e.
However, when melting of toner in the main nip portion is not maintained with much accuracy, uneven brightness occurs owing to unstable contact between the belt and the transfer medium.
Further, there is proposed a development that, for example, an elastic heat fixing roller 9FD is brought into contact with a pressure roller 9PD through the heat-resistant belt 9BD by pressurization, the heat fixing roller 9FD is distorted and the amount of the distortion is regulated not less than a predetermined value, and thereby a problem such as unevenness of image in separating the transfer medium is solved, as shown in FIG. 4D (JP Tokukaihei-5-150679A).
In recent years, the demands for duplex have increased, while it is necessary to separate a transfer medium having a fixing-completion toner image on one face and a not-fixed toner image on the other face after the transfer medium passes through the nip portion.
However, nothing about the separation performance in duplexing is described in the development shown in FIG. 4D.